How do I get crisp edges on Image Editor? (Get rid of fall off of stroke)
In the brush properties, go into the falloff menu. The curve you see is the intensity of the brush starting from the center and traveling outwards. Right now the curve is sloping downwards to the right, meaning that the intensity of the brush will be very intense in the middle, then will gradually decrease based on the curve. It will make more sense once you play with the curve a bit.
To change this, just change the graph so all the control points are on max height. Or, click the preset on the bottom which looks like a bracket. (It does the same thing)

(Something I drew)